1. Field of the Invention
The field of the invention is short circuit or fault current protection devices, and more particularly, circuits for protecting motor starters and the like from damage due to fault currents.
2. Background Art
The control circuitry for large electrical motors typically includes a starter consisting of a contactor and an overload relay. When closed, the contactor contacts permit the flow of current to the motor. The overload relay monitors the current to the motor and opens the contactor when necessary to safeguard the motor against overheating as may result when the motor is overloaded.
A typical overload relay is comprised of a heater and a thermal actuator such as a bimetallic strip or eutectic element. The heater reproduces, on a lesser scale, the heating of the motor itself and when a certain temperature is reached, the thermal actuator opens the overload relay contacts which in turn causes the contactor contacts to open and interrupt current to the motor.
It is known to place a circuit breaker or fuse in series with the starter to interrupt short circuit currents flowing through the starter. Circuit breakers are typically more complex than contactors and, like fuses, may handle such high short circuit currents.
Nevertheless, in the event of short circuit, the current through the contactor contacts and overload relay may exceed by twenty times or more the normal operating currents for those devices. This may occur if the current trip point of the circuit breaker or fuse is too high, or if sufficient energy is transmitted to the starter as a result of "let-through" current passed by the arc generated as the circuit breaker or fuse interrupts the circuit.
Under such high short circuit currents, the contactor contacts experience an electrodynamic "blow apart" force tending to open the contacts against the force of their actuator. When the contacts open, the high currents can cause arcing and pitting of the contact surfaces, destroying the operability of the contactor. The high currents can also damage the heater element of the overload relay. Hereinafter, the elements of a starter, or other such devices, that are subject to damage from the high current levels associated with a short circuit will be termed "fault current intolerant elements".